Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug insulator becomes coated with a foreign substance, such as fuel, oil, or soot. Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion until the spark plug is sufficiently cleaned or changed.
In areas with poor fuel quality control, spark plug fouling and pre-ignition caused by hot spark plugs is a significant issue. Fuel additives such as methycyclopentadienyl manganese tricarbonyl (MMT), lead or ferrocene may build up electrically conductive and thermally insulating deposits on the spark plug ceramic. Such build up may cause pre-ignition (PI), and consequently engine damage. The build-up can also cause misfires. The fuel additives can also coat combustion chambers leading to an increase in cylinder compression ratio and higher occurrence of combustion knock cycles. Further, the accumulation of fuel additive may not be easily removed.
In one example, the issues associated with spark plug fouling and the related pre-ignition can be addressed by a method for inferring spark plug fouling due to accumulation of fuel additives thereon. The early detection enables appropriate mitigating steps to be taken in a timely manner, thereby pre-empting engine degradation. One example method comprises: inferring spark plug fouling due to accumulation of fuel additive based on a combination of two or more of a change in an adaptive knock term, an engine pre-ignition rate, an exhaust oxygen sensor degradation rate, and exhaust catalyst degradation rate over a vehicle drive cycle. In this way, spark plug fouling from additives can be more reliably identified and accordingly addressed.
As an example, over a vehicle drive cycle, a controller may monitor a plurality of parameters correlated with spark plug health. These may include changes in an adaptive knock term, engine pre-ignition rate, engine misfire rate, engine exhaust catalyst degradation rate, and an exhaust oxygen sensor degradation rate. For example, the controller may monitor the change in spark angle retard applied at a given engine speed-load operating point over the vehicle drive cycle. The adaptive knock term may be monitored over a predetermined duration (e.g., time or number of engine cycles) of engine operation or predetermined distance of vehicle travel. In response to an increase in knocking rates with a progressively larger change in the adaptive knock term, spark plug fouling may be suggested. As another example, the controller may monitor an engine pre-ignition rate over a predetermined duration (e.g., time or number of engine cycles) of engine operation or predetermined distance of vehicle travel. If the pre-ignition rate is higher than a threshold rate (e.g., a threshold rate based on the engine's pre-ignition history), spark plug fouling may be suggested. As yet another example, the controller may monitor an engine misfire rate over a predetermined duration (e.g., time or number of engine cycles) of engine operation or predetermined distance of vehicle travel. If the misfire rate is higher than a threshold rate (e.g., a threshold rate based on the engine's misfire history), spark plug fouling may be suggested. As still another example, the controller may monitor a degradation rate of an exhaust oxygen sensor located upstream of an exhaust catalyst. In one example, the exhaust oxygen sensor degradation rate may be based on a switching frequency or response time of the exhaust oxygen sensor, the degradation rate increasing for decreasing switching frequency. As another example, the controller may monitor a degradation rate of an exhaust catalyst. In one example, the exhaust catalyst degradation rate may be based on a switching frequency or response time of the exhaust gas oxygen sensor located upstream of the exhaust catalyst and a switching frequency or response time of an exhaust gas oxygen sensor located downstream of the exhaust catalyst. If the switching frequencies or response time of the pre-catalyst and post-catalyst oxygen sensors are less than a threshold difference from each other, or if a ratio of the pre-catalyst to post-catalyst switching frequency or response time is higher/lower than a threshold, spark plug fouling may be suggested.
As such, if only one of the parameters is affected, the controller may determine that the spark plug is not fouled due to accumulation of fuel additives and may possibly be fouled due to accumulation of soot. Accordingly, mitigating steps may be taken to burn off the soot and clean the spark plug. However, if multiple (e.g., two or more) of the monitored parameters are affected, then the controller may infer that the spark plugs has fouled due to accumulation of fuel additives, rather than due to soot. The multiple monitored parameters may have weighting factors associated with them so that they have to occur together, some parameters carrying a heavier weight to due being affected by fuel additives more than others. Accordingly, mitigating steps may be taken to reduce fouling induced pre-ignition, such as by decreasing engine load, deactivating fuel to one or more engine cylinders, etc. In addition, a diagnostic code may be set to recommend spark plug replacement.
In this way, by reliably identifying spark plug fouling due to fuel additives, spark plug fouling induced pre-ignition may be reduced and timely mitigated. By monitoring multiple parameters whose change can be associated with spark plug health, spark plug fouling due to fuel additive accumulation can be accurately deduced without needing to rely only on complex and costly approaches (e.g., switching current measurements). By monitoring changes in the output of pre- and post-exhaust catalyst air-fuel ratio sensors, catalyst degradation due to fuel additive accumulation may also be timely identified and addressed. By providing spark plug replacement recommendations based on evidence of malfunction or degradation, rather than a predetermined period of time or amount of vehicle usage, spark plug change recommendations may not be provided too soon, lowering overall vehicle operational costs for the driver, while also reducing the risk of engine pre-ignition. By diagnosing spark plug health, engine life is extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.